In the oldest methods of forming a continuous paper or board web, which are still most commonly applied, forming of the web takes place in a horizontal so-called fourdrinier wire section, or a planar wire section. In addition, two main groups may be distinguished among the wire sections, or the former types, namely actual twinwire formers and so-called hybrid formers. In the actual twin-wire formers, forming of the web takes place from beginning to end between two wires. In the hybrid formers, the web is formed first onto one wire, after which this partly formed web is passed to a dewatering zone that is being formed between two wires for final stabilization of the mutual position of fibres.
One problem in the formers known from prior art is rewetting of the web. For instance, when a suction box of high vacuum is used in a former, even after the box there still occurs rewetting of the web, which is very difficult to prevent, before the web is transferred to a press section. On the other hand, it is known that wires all the time carry water in an amount of their own mass under normal running conditions, which water tends to move to the web if there is no force or another arrangement to prevent it.
Conventionally, vacuum zones, for instance, in suction rolls have been connected so as to be rising e.g. the first chamber of 25 kPa and the second chamber of 66 kPa in the case where attempts are made to increase the solids proportion without clogging up the web. A suction box of high vacuum is sought to be placed so that it is the last dewatering element in the former. It is known that solids may be increased by a high vacuum, but it is difficult to maintain the obtained solids.
With respect to prior art, reference may be made to the magazine article of Szikla, Palokangas, Role of felt in wet pressing, Paperi ja Puu 73, 1991, reporting an investigation of press felts in which it was observed that a vacuum of 20-40 kPa is enough to hold the water in the felt after a nip, thereby minimizing rewetting in a press. This result has been obtained with modern felts, in which the resistance to flow is small.
With respect to prior art, reference may also be made to EP Patent Applicaton No. 0 371 786, which discloses a web forming method and apparatus, the method disclosed being applied in a wire section of a paper machine or equivalent, which wire section is formed by a bottom wire loop with the main portion of its top run being substantially horizontal and a top wire loop cooperating therewith, and in which method a fibre suspension coming out of a headbox of the paper machine is supplied to the first part of the top run of said bottom wire loop, said part forming a first dewatering zone, after which the fibre layer from which part of the water has been removed is passed to a second dewatering zone, in the area of which said top wire loop is caused to cover said fibre layer, from which part of the water has been removed, such that the removal of water from the fibre layer continues at least in two stages in the area of said second dewatering zone, after which the top wire loop is separated from the nearly formed fibre web, which is guided to follow the run of the bottom wire loop forwards for the following processing stages of the web. In connection with this prior-art arrangement a separation suction box has been used in the downwards slanting run of the bottom wire between a forming roll and a suction roll, the purpose of said suction box being to ensure that the web does not follow the wire, which returns to the wire section, but is transferred from the wire section to a press section. In this known arrangement the web, supported by the bottom wire and ensured by the suction box, moves over the suction roll to the run between it and the drive roll of the wire, from which the web is transferred, in a way known in itself, by means of a pick-up roll and a felt forwards to the press section of the paper machine. However, in this arrangement no attention has been paid to prevention of rewetting.